build/oci: look up subgid from username

the subgid maps are not mapped per group, but per user

Change-Id: Iaf367cff159109eefb5ef99d58033082e6151c5d
Reviewed-on: https://cl.snix.dev/c/snix/+/30568
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Reviewed-by: Vova Kryachko <v.kryachko@gmail.com>

Author: yuyuyureka

snix/build/src/oci/subuid.rs

@@ -55,11 +55,8 @@ impl SubordinateInfo {
 
         let subuid =
             first_subordinate_id(&PathBuf::from("/etc/subuid"), user.uid.as_raw(), &user.name)?;
-        let subgid = first_subordinate_id(
-            &PathBuf::from("/etc/subgid"),
-            group.gid.as_raw(),
-            &group.name,
-        )?;
+        let subgid =
+            first_subordinate_id(&PathBuf::from("/etc/subgid"), user.uid.as_raw(), &user.name)?;
         Ok(SubordinateInfo {
             uid: user.uid.as_raw(),
             gid: group.gid.as_raw(),